Byrne provided an explanation for heat vision, too. It was in Action Comics, I think, when he was going through the team-up stories and had Supes interacting with the GLC. One of the GL's observed Kal's heat vision and theorised that it was a combination of Kal's vision powers (I can't recall the exact wording) and that Kal was agitating the molecules (via telekinesis).
I'll try to pull up the issue later today or something.